Best endgame drills / software?

My son whose account this is wants endgame practice. It needs to be on Ipad (he has bus time, etc, that he wants to use). He's about 1700 OTB.  We've been recommended several resources, wanted specific reactions from people who have had success hands-on drilling on endgames as they push toward the 1800 - 1900 level.

- ChessKing -- Total Endings IOS (this looks very thorough)

- Get 100 endgames to know book on Chessable

- Chess.com drills 

Thanks!

- ChessTempo 

I use ChessTempo

1. Chessable: 100 Endgames You Must Know

2. ChessKing: Total Chess Endgames (1600-2400 ELO)

If you can take both. With ChessKing I'd recommend Ultimate edition, then you have Web+Android+iOS.

But the most important for endgame training: play all examples against an engine. It will often go into side variations that are not in the book, that's where your understanding and analytical skills are trained.